This invention relates to methods and systems for steam electrolysis. In particular, the invention relates to the conversion of waste materials into hydrogen. This invention further relates to reversible systems and methods for steam electrolysis and energy generation using solid oxide technology.
Steam reforming is a process that involves reaction of methane and/or other hydrocarbons with steam at temperatures between 700-1300K over a nickel catalyst on a ceramic substrate. The reaction results in producing primarily carbon monoxide and hydrogen with small amounts of residual hydrocarbons and impurity reaction byproducts such as oxides of sulfur and nitrogen. The primary reaction in the steam reforming process is:CH4((g))+H2O((g))=CO((g))+3H2((g))
The traditional steam reforming process does not allow clean separation of hydrogen from carbon monoxide and the other impurity oxides that are generated in the process in a single step.
A higher purity hydrogen gas may be formed using steam electrolysis processes. In the electrolysis process hydrogen is generated by applying electrical energy to split water or steam. Typically two electrodes are used, hydrogen is generated at the cathode and oxygen at the anode. This process is energy intensive and expensive but can produce clean and pure hydrogen.
Mixed ionic and electronic conducting (MIEC) membranes have recently been considered for a wide variety of gas separation applications including oxygen separation, partial oxidation of methane, and hydrogen separation. In one example of this process, one side of an oxygen ion and electron conducting MIEC membrane is exposed to steam and the other side to a hydrocarbon such as methane. This sets up a chemical potential gradient in O2 across which transport of oxygen occurs from the steam side to the hydrocarbon side leaving behind a H2 rich product on the steam side and a product rich in synthesis gas (syn-gas) on the hydrocarbon side of the membrane. Hydrogen separation and purification using MIEC membranes are described in published PCT application WO 03/089117, which is incorporated in its entirety by reference. This process produces pure hydrogen and syn-gas from a source of steam and hydrocarbon fuel, however, additional processing is required to obtain the syn-gas used in this process.
Synthesis gas (syn-gas) is the name given to gases of varying composition that are generated in coal gasification and consists primarily of carbon monoxide and hydrogen. Syn-gas is typically prepared using a gasification process.
Gasification is a process that converts carbon-containing materials, such as coal, petroleum, petroleum coke or biomass, into carbon monoxide and hydrogen. In a gasifier, the carbonaceous material undergoes three processes, pyrolysis, combustion and gasification. During pyrolysis, the volatiles from the carbonaceous particle are removed by heating and the residue remaining forms the char. During combustion, the volatiles and some of the char react with oxygen to form carbon monoxide and carbon dioxide. This reaction also produces heat. During gasification, the char and carbon dioxide react with steam to produce carbon monoxide and hydrogen. Pyrolysis, combustion and gasification process releases large amounts of environmental pollutants that are present in coal including heavy metals and oxides of nitrogen, sulfur, hydrocarbon and carbon. Clean separation of hydrogen from other gases and impurities is a complex, difficult, and expensive process.
Another way to generate hydrogen is through incineration, which converts waste matter to other more acceptable forms of matter by heating it to a very high temperature (greater than thousand degrees centigrade). The process can decompose the waste matter or can selectively remove certain constituents in the matter by taking advantage of different boiling and flash points. In the incinerator the waste undergoes combustion reactions producing ash and combustion gases. The products of incineration are often further treated before release to meet regulatory standards for waste disposal. The incineration process is usually used as an alternative to landfilling and bioremediation, because it drastically reduces the volume of solid waste. The thermal energy released in the incinerator is often utilized in other processes. The waste incineration process however does not produce pure hydrogen and/or syn-gas. It does, however, serve to generate thermal energy that can be used in other processes, e.g., steam generation.
A fuel cell is an electrochemical device that converts the chemical energy in fuels (such as hydrogen, methane, butane or even gasoline and diesel) into electrical energy by exploiting the natural tendency of oxygen and hydrogen to react. Much development has focused on solid oxide fuel cells (SOFC), both because they are able to convert a wide variety of fuels into energy and because they do so with high efficiency. High efficiency and fuel adaptability are not the only advantages of solid oxide fuel cells. SOFCs are attractive as energy sources because they are clean, reliable, and almost entirely nonpolluting. Because there are no moving parts and the cells are therefore vibration-free, the noise pollution associated with power generation is also eliminated.
There is a great present and future need for pure hydrogen to power fuel cells for transportation, defense, and consumer electronic applications. There is also a large demand for syn-gas for stationary and distributed power generation. There remains an unmet need to generate efficient, low cost methods for generating high purity hydrogen.